1. Field of the Invention
This invention relates to a vacuum processing apparatus suitable for various working processes utilizing vacuum, and more particularly to a vacuum processing apparatus suitable for forming a thin film on a substrate to be processed by the use of the plasma CVD (chemical vapor deposition) method.
2. Description of the Prior Art
In recent years, attention has been paid to the reactive sputtering method and the plasma CVD method as a thin film forming method. For example, the plasma CVD method is such that the pressure in a reaction chamber is reduced to a desired degree of vacuum and a gas as a raw material is supplied to the reaction chamber, whereafter the gas is decomposed by glow discharge or are discharge and a thin film is formed on a substrate disposed in the reaction chamber. Amorphous silicon (hereinafter referred to as "a-Si" formed by this method with silane (SiH4 or Si2H6) gas as the raw material, the localized level present in the inhibited zone of a-Si is relatively small in number, and as the result, it becomes capable to control electrons by the doping of substitution type impurities. Therefore the film can be used also as an electrophotographic photosensitive medium.
FIG. 1 of the accompanying drawings shows an example of a plasma CVD apparatus according to the prior art and more particularly, it is a side cross-sectional view of a typical batch type plasma CVD apparatus for forming an Si layer on a cylindrical substrate 3 disposed in a single vacuum tank body 10 adapted to be hermetically sealed by a lid 1. A rotational mechanism 5 driven by a motor M is disposed at the center of the bottom of the vacuum tank body 10, and the cylindrical substrate 3 heated by an internal heater 4 is fixedly placed on the rotational mechanism 5. The heating by the internal heater 4 is necessary to maintain the substrate 3 at a high temperature (e.g., 250.degree. C.) in order to form a stable a-Si layer on the substrate 3.
A cylindrical cathode electrode 2 connected to a high-frequency power source 7 is disposed around the substrate 3, and a cylindrical shield 6 for confining plasma gas is disposed around the electrode 2. A gas introducing system 8 for introducing raw material gas (silane gas) into the cylindrical shield 6 constituting a reaction chamber has its gas inlet disposed at the bottom of the shield 6, and an exhaust system 9 for making said reaction chamber vacuum has its exhaust port disposed at the bottom of the vacuum tank body 10.
In this apparatus, when the substrate 3 is heated and the raw material gas is introduced upwardly from below in the shield 6 of the reaction chamber, a high frequency electric power is applied from the power 7 to the cathode electrode 2 and plasma a discharge is produced between the electrode 2 and the substrate 3 which is an anode electrode, and the silane gas is decomposed and accordingly, an a-Si film is formed on the surface of the substrate.
The remaining gas after the reaction is discharged from an aperture 6a formed in the top of the shield 6, moves down between the outer wall of the shield 6 and the inner wall of the vacuum tank body 10 and is exhausted outwardly of the apparatus by the exhaust system 9.
However, in the above-described plasma CVD apparatus according to the prior art, only one substrate can be formed with a film by one reaction furnace, and this leads to poor working efficiency, that is, a disadvantage that this apparatus is unsuitable for mass production.
Further, the reaction furnace in the apparatus according to the prior art is exposed to the atmosphere each time the product formed with a film is removed and therefore adsorbs the moisture or gases in the atmosphere, and this leads to an inconvenience that the quality of the film is reduced or becomes unstable.
On the other hand, a so-called load lock type vacuum apparatus has been devised to eliminate these disadvantages. This apparatus, as shown in FIG. 2 of the accompanying drawings, comprises a substrate receiving chamber 21, a working process chamber 22 and a substrate removal chamber 23. In the substrate receiving chamber 21, preliminary processings such as the processing of the substrate between vacuum and the atmosphere and heating of the substrate, and then the substrate is moved into the working process chamber 22 through a gate 24 provided between the substrate receiving chamber 21 and the working process chamber 22 while the two chambers are kept in their vacuum state. Then, the substrate already formed with a film in the working process chamber 22 is transferred through a similar gate 24 provided between the working process chamber 22 and the removal chamber 23 and is subjected to processing such as cooling in the removal chamber 23, whereafter the atmosphere is introduced into the removal chamber and the substrate formed with a film is removed.
The apparatus of this type is efficient as compared with the apparatus of the former type, but it is not suitable as an apparatus for making an amorphous silicon electrophotographic photosensitive medium or the like.
That is, the above-described load lock type vacuum processing apparatus according to the prior art suffers from the following problems. Firstly, each vacuum tank becomes bulky to contain therein a number of large substrates such as photosensitive mediums and thus, the cost of the apparatus becomes high. Secondly, the heating of the substrate is effected from the inside of the cylindrical substrate and therefore, the conveying mechanism for the cylinder becomes complex.